With the recent pervasion of personal information terminals using a computer or flat display panel, there has been rapid development of technologies involving integrated circuit element, TFT (Thin Film Transistor)-liquid crystal display, or TFT-organic EL display.
For example, in the integrated circuit element technology, a commercially-available monocrystalline Si (silicon) wafer, circular in shape with a thickness of less than 1 mm and a diameter of about 20 mm, is processed and about several hundred million transistors are formed on the monocrystalline Si wafer.
The TFT-liquid crystal display or TFT-organic EL display technology now employs a technique in which pixels or drivers of the liquid crystal display are made first by forming an amorphous Si film (“a-Si” hereinafter) or a polycrystalline semiconductor film such as a polycrystalline Si film (“p-Si” hereinafter) on a light-transmissive amorphous substrate such as a glass substrate, and then by processing such a film into a thin film transistor (“TFT” hereinafter). For example, display devices that drive the liquid crystal display, organic EL display, and the like by a method of driving generally known as active-matrix driving have been used, wherein the active-matrix driving is realized by the MOS transistor, used as a switching element, which is processed out of an amorphous Si film deposited on a high-strain-point glass substrate. The MOS transistor is formed by melting the amorphous Si film with the heat of a heat source such as a laser, so as to achieve a polycrystalline state in the amorphous Si film.
Particularly common is one with integrated peripheral drivers using p-Si that offers fast operation by its high mobility.
For the system integration of high-performance devices such as an image processor or timing controller, there is a demand for a Si device with better performance.
The need for better performance arises from the insufficient performance of the transistor for making a high-performance Si device, owning to the fact that the mobility is decreased or S coefficient (sub-threshold coefficient) is increased by the presence of a local level in the gap caused by the incomplete crystallinity of the polycrystalline Si, or by the presence of a defect or such a gap local level in the vicinity of a crystal grain boundary.
In light of such a drawback, there has been active research since the 1980's on a so-called SOI (silicon on insulator) technique, in which a monocrystalline Si thin film is formed on a substrate whose amorphous portion is in contact with the Si film. Note that, as the term is used herein, “SOI” refers to the act of forming the monocrystalline Si film on an insulating substrate, or a structure in which the monocrystalline Si film is formed on an insulating substrate. (Generally, the term “SOI” is used when the monocrystalline Si film is formed as a Si layer.)
In the field of integrated circuit, the SOI substrate is used to make desirable transistors and thereby drastically improve the functions of the semiconductor element. Generally, the type of substrate on which the monocrystalline Si film is formed in the integrated circuit is not particularly limited as long as it is an insulator. The substrate may be transparent or non-transparent, or crystalline or amorphous. Further, with the elements completely isolated, the integrated circuits have less restriction in term of operation. Thus, by making the transistor using the SOI substrate, desirable characteristics and high performance can be realized at the same time.
For improved performance of the Si device, a technique of making a semiconductor device has been researched, in which the semiconductor device is formed by first processing a monocrystalline Si thin film into a device such as a thin film transistor, and then by bonding the thin film transistor on an insulating substrate. (For example, see International Publication No. WO93/155898, J. P. Salerno “Single Crystal Silicon AMLCDs”, Conference Record of the 1994 International Display Research Conference (IDRC), pp. 39-44 (1994), Q.-Y. Tong & U. Gesele, SEMICONDUCTOR WAFER BONDING: SCIENCE AND TECHNOLOGY, John Wiley & Sons, New York (1999)).
For example, Japanese Publication for Unexamined Patent Application No. 17107/1999 (Tokukaihei 11-17107, published on Jan. 22, 1999) (Publication 1) discloses a technique in which a two-dimensional LSI made from a monocrystalline silicon layer is formed on a porous silicon layer formed on a monocrystalline silicon substrate, and a supporting substrate is bonded to a surface of the two-dimensional LSI, before the two-dimensional LSI is detached from the monocrystalline silicon substrate in portions of the porous silicon layer.
In the SOI device, the monocrystalline Si is solely formed on the Si wafer having an insulating film, as disclosed in this publication. In fabricating a latch-up free or completely depleted element, this is advantageous over the bulk monocrystalline Si element that is directly made out of a Si wafer. However, due to chip size restriction etc., the applicable area of the SOI device has been limited to the fields of ICs and LSIs, including the semiconductor memory element.
This makes the use of the SOI device solely made of monocrystalline Si difficult when the semiconductor device is to be formed on a large-area light-transmissive amorphous substrate as in the active-matrix display device. In order to avoid such a drawback, the active-matrix display device generally adopts a structure in which a non-monocrystalline Si device is formed on a large-area light-transmissive substrate such as a glass substrate.
However, when used alone, it is extremely difficult for the non-monocrystalline Si device to match its performance with that of the monocrystalline Si device. For example, a non-monocrystalline Si thin film transistor (low-temperature polycrystalline Si thin film transistor, etc.) formed on a high-strain-point glass substrate by being crystallized by irradiation of an energy beam has incomplete crystallinity and therefore it can achieve a mobility of only about 300 cm2/VS even when it is an NMOS transistor. This arises from the insufficient performance of the transistor for making a high-performance Si device, owning to the fact that the mobility is decreased or S coefficient (sub-threshold coefficient) is increased by the presence of a local level in the gap caused by the incomplete crystallinity of the polycrystalline Si, or by the presence of a defect or such a gap local level in the vicinity of a crystal grain boundary.
That is, while the common method of depositing the amorphous Si film and crystallizing it into a polycrystal by irradiation of an energy beam may be sufficient to raise the performance of the Si film to the level of monocrystalline Si, it is insufficient to exactly match the performance of the Si film to that of the monocrystalline Si film. Thus, in order to obtain the very same performance of the monocrystalline Si device with the non-monocrystalline Si device alone, a further technical breakthrough is needed, requiring many more stages of development.
Note that, the source driver (data driver) of the active-matrix display device can still meet the requirements of required performance as a source driver even when devices of the same material are monolithically mounted on a single glass substrate. However, for devices (controller, D/A converter, etc.) that need to satisfy tougher requirements for their characteristics, it is difficult to monolithically mount devices of the same material on a single glass substrate and still obtain uniform threshold voltages and uniform elements with high mobility. That is, required characteristics cannot be obtained.
As another method of forming a high-performance semiconductor device, a method is available in which two kinds of semiconductor devices with different characteristics are formed on a single substrate. For example, Japanese Publication for Unexamined Patent Application No. 24106/1999 (Tokukaihei 11-24106, published on Jan. 29, 1999) (Publication 2) discloses a technique in which a substrate for a liquid crystal panel is fabricated by transferring polycrystalline silicon TFTs from one substrate to another substrate on which pixel regions with amorphous silicon TFTs have been formed.
It should be noted here that the foregoing publications 1 and 2 assume a construction in which a monocrystalline Si device and a polycrystalline Si device are formed on a single insulating substrate, wherein the former has been transferred from a different substrate, and the latter has been deposited on the insulating substrate. In this case, the polycrystalline Si thin film is formed by turning the amorphous Si thin film to a polycrystal by irradiation of a laser beam.
There are two ways to form the two kinds of semiconductor devices, the monocrystalline Si device and the polycrystalline Si device, on the insulating substrate. The first method is to first transfer the monocrystalline Si device on the insulating substrate and then form the polycrystalline Si device. The second method is to first form the polycrystalline Si device on the insulating substrate and then transfer the monocrystalline Si device.
By comparing these two methods, the advantage of the first method in which the polycrystalline Si device is formed after the monocrystalline Si device is transferred is that it allows the monocrystalline Si device to be transferred onto a flat surface of the insulating substrate and thereby prevent the problem of contact failure, etc. However, a drawback of this method is that the monocrystalline Si device is damaged as the laser that irradiates the amorphous Si thin film in the step of turning the amorphous Si thin film to a polycrystal also irradiates the monocrystalline Si device.
More specifically, the energy of the irradiated beam for achieving a polycrystalline state damages the monocrystalline Si device, making it difficult to obtain uniform threshold voltages and uniform elements with high mobility.